Method for producing acetate grade dissolving pulp from poplar residual slabs

ABSTRACT

The disclosure relates to the technical field of dissolving pulp preparation, and particularly relates to a method for producing acetate grade dissolving pulp utilizing poplar residual slabs. The method comprises the following steps: adding an auxiliary agent for prehydrolysis, performing a Soda/AQ cooking I segment, performing a Soda/AQ cooking II segment, performing OpD Q E P P bleaching and performing enzyme coprocessing, thereby preparing high-purity high-reactivity poplar residual slab acetate grade dissolving pulp. In the method, used chemicals are clean and environmental friendly, and production process is clean.

TECHNICAL FIELD

The disclosure pertains to the technical field of dissolving pulp preparation, and specifically pertains to a method for producing acetate grade dissolving pulp utilizing poplar residual slabs.

BACKGROUND

At present, the yield of cellulose acetate is approximately one million tons in the world, and the yield of cellulose acetate is almost 400 thousand tons in China, about 300 thousand tons of acetate grade dissolving pulp is consumed yearly, the acetate grade dissolving pulp is mainly used for preparing cellulose triacetate and cellulose diacetate, hydroxyl groups at 2, 3, 6 positions of an anhydroglucose unit (AGU) in cellulose are almost completely acetylated in the dissolving pulp to obtain cellulose triacetate (substitution degree >2.7), cellulose triacetate is hydrolyzed to obtain cellulose diacetate (substitution degree is 2.4-2.6), and the reaction process is as shown in a formula as follows.

Cellulose triacetate is widely applied to films, liquid display polarizer films, reverse osmosis membranes and the like, and cellulose diacetate is widely applied to cigarette filters, plastic products and the like. Cellulose acetate has high requirements on its raw material-acetate grade dissolving pulp, namely, high purity and high reactivity. At present, acetate grade cotton pulp can only be prepared in China, and high-purity high-reactivity acetate grade wood dissolving pulp mainly depends on imports.

At home and abroad, acetate grade dissolving pulp is prepared using woods as a raw material, and a process typically adopts prehydrolysis, sulfate cooking, ECF bleaching or acidic sulfite cooking, ECF bleaching and cold alkali extraction. In order to improve the removal of hemicellulose in the prehydrolysis process, inorganic acid or organic acid is typically added, which increases corrosivity of equipment. In order to improve a delignification rate, some researchers adopt a high-sulfidity KP cooking technology, however, high sulfidity easily causes generation of odors such as methyl mercaptan and dimethylsulfide; in order to reduce the ash content of the dissolving pulp, a dissolving pulp production enterprise need to perform acidic treatment after pulp is bleached, however, a low pH value easily causes polymerization decrease of partial celluloses in the dissolving pulp due to acidic hylydrosis; in order to improve the content of α-cellulose in the acetate grade dissolving pulp, some scholars adopt cold alkali extraction, but it severely reduces the reactivity of the acetate grade dissolving pulp and acetic anhydride, and some scholars in Egypt adopt microwave radiation and high-concentration zinc chloride to treat the acetate dissolving pulp so as to improve its reactivity, however, the technology is not matured; a U.S. patent reports that strong acid is used to treat the acetate dissolving pulp so as to improve its purity, but this technology is high in pollution degree and high in cost. Several institutions in China are researching and developing this technology, mainly including South China University of Technology, Beijing Forestry University and other departments which have gained some progresses in research and development of this technology, mainly focusing on preparation of acetate grade dissolving pulp using eucalyptus, acacia and pine.

As a wood processing residue, an poplar residual slab is low in cost and sufficient in raw material. It is reported that the production amount of wood processing residue is about 50 million tons yearly in China. As one of wood processing residues, the poplar residual slab has holocellulose content of about 82%, among them, the content of α-cellulose is about 44%. If high-reactivity acetate grade dissolving pulp is produced with the poplar residual slab as a raw material, development of the raw material of the acetate grade dissolving pulp will be greatly facilitated. However, there are no any reports about preparation of the acetate grade dissolving pulp using the poplar residual slab at home and abroad.

SUMMARY

In order to solve the technical blank of the prior art that preparation of acetate grade dissolving pulp using poplar residual slabs is not present, the present application discloses a method for producing acetate grade dissolving pulp using poplar residual slabs.

By utilizing a prehydrolysis technology in which an auxiliary agent is added, cellulose is selectively protected while removing hemicellulose in the poplar residual slab raw material and fiber cells are swelled; then lignin and hemicellulose are selectively removed adopting segmented cooking, and polymerization decrease of cellulose is controlled; after cooking, pulp is subjected to oxygen delignification enhanced with hydrogen peroxide/chlorine dioxide bleaching with addition of a chelating agent/hot alkali extraction with addition of hydrogen peroxide/residual lignin and hemicellulose removal via hydrogen peroxide bleaching (O_(p)D_(Q)E_(P)P), the brightness of the pulp is improved and the degree of polymerization of the dissolving pulp is regulated, and then the bleached pulp is subjected to biological enzyme coprocessing to further improve its acetifying reaction value (PV value), reduce ash content and dichloromethane (DCM) extract content. After using this process, the purity and reactivity of the acetate grade dissolving pulp are improved.

The disclosure is obtained by the following steps:

Provided is a method for producing acetate grade dissolving pulp utilizing poplar residual slabs, comprising the following steps:

(1) adding an auxiliary agent for prehydrolysis: a liquid ratio is 1:(4˜10), the addition amount of a hydrolysis auxiliary agent is 0.1˜2.0% of the mass of dry poplar residual slab, the hydrolysis temperature is 80˜170° C., preservation time is 10˜180 minutes, the hydrolysis auxiliary agent is a mixture obtained by mixing sodium borate, magnesium sulfate, nonylphenol polyoxyethylene ether phosphate and tetrasodium iminodisuccinate in a mass ratio of 1:1:1:1;

(2) performing a Soda/AQ cooking I segment;

(3) performing a Soda/AQ cooking II segment;

(4) performing O_(p)D_(Q)E_(P)P bleaching; and

(5) performing enzyme coprocessing: the concentration of pulp is 2˜10%, the temperature is 30˜50° C., treatment time is 60 min, pH=4.0˜7.0, the amount of polyaspartic acid is 0.01%˜1.0% of the mass of oven dry pulp, the amount of OP-13 is 0.001˜0.01% of the mass of oven dry pulp, the amount of OP-15 is 0.001˜0.01% of the mass of oven dry pulp, the amount of endoglucanase is 0.1˜0.6 IU/gpulp, the amount of xylanase is 2˜4 XU/gpulp, and the amount of pectase is 0.1˜10 IU/gpulp.

The preferred process conditions of the Soda/AQ cooking I segment are as follows: the amount of alkali is 5˜15% of the mass of oven dry pulp based on Na₂O, a liquid ratio is 1: (3˜8), the amount of magnesium hydroxide is 0.5% of the mass of oven dry pulp, the amount of sodium sulfite is 1.0˜10.0% of the mass of oven dry pulp, the amount of AQ is 0.05% of the mass of oven dry pulp, the temperature is 80˜120° C., and preservation time is 10˜180 minutes.

The preferred process conditions of the Soda/AQ cooking II segment are as follows: the amount of alkali is 5˜25% of the mass of oven dry pulp based on Na₂O, the amount of magnesium hydroxide is 0.5% of the mass of oven dry pulp the amount of sodium sulfite is 1.0˜10.0% of the mass of oven dry pulp, a liquid ratio is 1:(3˜10), the amount of AQ is 0.05% of the mass of oven dry pulp, the temperature is 140˜180° C., and preservation time is 10˜240 minutes.

The preferred process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the oxygen pressure of O_(p) bleaching is 0.4˜0.6 Mpa, the concentration of pulp is 10%, the amount of MgSO4 is 0.6% of the mass of oven dry pulp, the amount of NaOH is 1.0˜3.0% of the mass of oven dry pulp, the amount of H₂O₂ is 0.5˜3.0% of the mass of oven dry pulp, the temperature is 70˜110° C., and preservation time is 60 min.

The preferred process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the reaction time of a Q_(D) bleaching process is 10˜120 minutes, T=30±2° C., the concentration of pulp is 2˜8%, pH=1˜3, the amount of sodium chlorite is 0.2˜6.0% of the mass of oven dry pulp, and the amount of IDS is 0.01˜1.0% of the mass of oven dry pulp.

The preferred process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the reaction time of the E_(p) extraction process is 30˜180 min, the amount of MgSO₄ is 0.5% of the mass of oven dry pulp, the amount of NaOH is 0.5˜5.0% of the mass of oven dry pulp, the amount of H₂O₂ is 1.0˜5.0% of the mass of oven dry pulp, the amount of IDS is 0.01˜1.0% of the mass of oven dry pulp, the concentration of pulp is 8%, and the temperature is 70˜95° C.

The preferred process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the reaction time of P bleaching is 60˜180 min, the amount of MgSO₄ is 0.5% of the mass of oven dry pulp, the amount of NaOH is 0.4% of the mass of oven dry pulp, the amount of NaAC is 0.4% of the mass of oven dry pulp, the amount of H₂O₂ is 1.0˜5.0% of the mass of oven dry pulp, the concentration of pulp is 10%, and the temperature is 80˜95° C.

In summary, hemicellulose in the acetate grade dissolving pulp is selectively removed by adding an auxiliary agent when poplar residual slabs are prehydrolyzed, then Soda/AQ segmented cooking with addition of the auxiliary agent is carried out to remove lignin to the greatest extent so as to protect cellulose, the pulp is subjected to O_(p)D_(Q)E_(P)P bleaching after being cooked to further remove lignin and regulate the degree of polymerization of the dissolving pulp, and finally, by using an enzyme coprocessing process, the purity of the dissolving pulp is further improved and the reactivity of the dissolving pulp is regulated, thereby obtaining high-purity high-reactivity poplar residual slab acetate grade dissolving pulp.

The disclosure has the beneficial effects:

(1) at first, hemicellulose in the poplar residual slab is greatly selectively dissolved out by virtue of prehydrolysis with addition of an auxiliary agent, and finally the purity and the acetifying reaction value of the poplar residual slab pulp are improved using enzyme coprocessing, and then high-purity high-reactivity poplar residual slab acetate grade dissolving pulp is prepared by combining with Soda/AQ segmented cooking and O_(p)D_(Q)E_(P)P bleaching;

(2) the poplar residual slab is treated adopting the process of the disclosure, the used chemicals are clean and environmental friendly, and the production process is clean.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flowchart of acetate grade dissolving pulp produced by using poplar residual slab.

DESCRIPTION OF THE EMBODIMENTS

In order to better understand the disclosure, the disclosure will be further described in combination with specific examples next.

In the following examples, a hydrolysis auxiliary agent is a mixture obtained by mixing sodium borate, magnesium sulfate, nonylphenol polyoxyethylene ether phosphate and tetrasodium iminodisuccinate in a mass ratio of 1:1:1:1.

Percentage contents of materials used in various process stages are all percentage contents accounting for their oven dry pulp mass.

Example 1

A prehydrolysis with addition of an auxiliary agent-Soda/AQ segmented cooking-O_(p)D_(Q)E_(P)P-enzyme coprocessing process is adopted to treat a poplar residual slab raw material: 500 g of poplar residual slab chips from a factory were taken, with a length of 10˜30 mm, pentosan content of 24.3% and α-cellulose content of 43.0%.

The process conditions of prehydrolysis with addition of an auxiliary agent are as follows: a liquid ratio is 1:6, the auxiliary agent is 0.4%, the maximum temperature is 160° C., preservation time is 60 minutes, the content of pentosan after hydrolysis is 8.6%, and the hydrolysis yield is 80%.

The process conditions of a Soda/AQ cooking I segment are as follows: the amount of alkali is 5%, a liquid ratio is 1:4, magnesium hydroxide is 0.5%, sodium sulfite is 5.0%, the amount of AQ is 0.05%, T_(max)=100° C.±2° C., and preservation time is 60 minutes.

The process conditions of a Soda/AQ cooking II segment are as follows: the amount of alkali is 18%, magnesium hydroxide is 0.5%, sodium sulfite is 3.0%, a liquid ratio is 1:7, the amount of AQ is 0.05%, the maximum temperature is 160° C., and preservation time is 120 minutes.

The oxygen pressure of O_(P) bleaching is 0.5 Mpa, the concentration of pulp is 10%, MgSO₄ is 0.6%, NaOH is 2.0%, H₂O₂ is 1.0%, the temperature is 95° C., and preservation time is 60 min.

The reaction time of the Q_(D) bleaching process is 60 minutes, T=30±2° C., the concentration of pulp is 4.0%, pH=1.5, the amount of sodium chlorite is 3.0%, and the amount of IDS is 0.05%.

The reaction time of the E_(p) extraction process is 90 min, MgSO₄ is 0.5%, NaOH is 3.0%, H₂O₂ is 2.0%, the amount of IDS is 0.05%, the concentration of pulp is 8%, and the temperature is 85° C.

The reaction time of P bleaching is 120 min, MgSO₄ is 0.5%, NaOH is 0.4%, NaAC is 0.4%, H₂O₂ is 3.0%, the concentration of pulp is 10%, and the temperature is 90° C.

The conditions of enzyme coprocessing are as follows: the concentration of pulp is 5%, the temperature is 40° C., treatment time is 60 min, pH=5.0, the amount of polyaspartic acid is 0.05%, the amount of OP-13 is 0.005%, the amount of OP-15 is 0.005%, the amount of endoglucanase is 0.2 IU/gpulp, the amount of xylanase is 3 XU/gpulp, and the amount of pectase is 3 IU/gpulp.

The obtained poplar residual slab acetate grade dissolving pulp has α-cellulose content of 97%, pentosan content of 2.6%, a degree of polymerization of 1250, a brightness of 89% ISO, an ash content of 0.01%, DCM extract content of 0.03% and an acetifying reaction value (PV value) of 800.

The poplar residual slab acetate grade dissolving pulp is good in various detection indexes, and can be used for producing a high-grade cellulose acetate product.

Example 2

A prehydrolysis with addition of an auxiliary agent-Soda/AQ segmented cooking-O_(p)D_(Q)E_(P)P-enzyme coprocessing process is adopted to treat a poplar residual slab raw material, the taken poplar residual slab has a pentosan content of 22.3% and a α-cellulose content of 45.0%, and the process conditions are the same as those in Example 1. The obtained poplar residual slab acetate grade dissolving pulp has α-cellulose content of 97%, pentosan content of 2.2%, a degree of polymerization of 1280, a brightness of 90% ISO, ash content of 0.01%, a DCM extract content of 0.02% and an acetifying reaction value (PV value) of 820.

The poplar residual slab acetate grade dissolving pulp is good in various detection indexes, and can be used for producing a high-grade cellulose acetate product.

Example 3

A prehydrolysis with addition of an auxiliary agent-Soda/AQ segmented cooking-O_(p)D_(Q)E_(P)P-enzyme coprocessing process is adopted to treat a poplar residual slab raw material, and the taken poplar residual slab is the same as that in Example 1. The preservation time of prehydrolysis is only changed as 120 minutes, the content of pentosan after hydrolysis is 8.5%, the hydrolysis yield is 79%, and other processes are unchanged. The finally obtained poplar residual slab acetate grade dissolving pulp has α-cellulose content of 97% and pentosan content of 2.2%, a degree of polymerization of 1200, a brightness of 89% ISO, ash content of 0.01%, DCM extract content of 0.02% and an acetifying reaction value (PV value) of 780.

Example 4

A prehydrolysis with addition of an auxiliary agent-Soda/AQ segmented cooking-O_(p)D_(Q)E_(P)P-enzyme coprocessing process is adopted to treat a poplar residual slab raw material, and the taken poplar residual slab is the same as that in Example 1. The cooking liquid ratio of the Soda/AQ cooking II segment is only changed as 1:4 and other processes are unchanged. The finally obtained poplar residual slab acetate grade dissolving pulp has α-cellulose content of 97%, pentosan content of 2.5%, a degree of polymerization of 1600, a brightness of 90% ISO, ash content of 0.01%, DCM extract content of 0.02% and an acetifying reaction value (PV value) of 730.

The poplar residual slab acetate grade dissolving pulp is good in various detection indexes, and can be used for producing a high-grade cellulose acetate product.

Example 5

A prehydrolysis with addition of an auxiliary agent-Soda/AQ segmented cooking-O_(p)D_(Q)E_(P)P-enzyme coprocessing process is adopted to treat a poplar residual slab raw material, and the taken poplar residual slab is the same as that in Example 1. Prehydrolysis with addition of an auxiliary agent in the steps are removed, a liquid ratio is 1:6, and other processes are the same as those in Example 1. The finally obtained poplar residual slab acetate grade dissolving pulp has α-cellulose content of 95%, pentosan content of 3.6%, a degree of polymerization of 1200, brightness of 88% ISO, ash content of 0.015%, DCM extract content of 0.025% and an acetifying reaction value (PV value) of 680.

Example 6

A prehydrolysis with addition of an auxiliary agent-Soda/AQ segmented cooking-O_(p)D_(Q)E_(P)P-enzyme coprocessing process is adopted to treat a poplar residual slab raw material, and the taken poplar residual slab is the same as that in Example 1. Two segments of cooking in steps (2) and (3) are combined into one segment of cooking, alkali, magnesium hydrate, sodium sulfite and AQ used in two segments are added once, a liquid ratio is 1:6, T_(max)=100° C.±2° C., and preservation time is 60 minutes; then thermal preservation is performed for 120 minutes at 160° C., and other processes are the same as those in Example 1. The finally obtained poplar residual slab acetate grade dissolving pulp has α-cellulose content of 96% and pentosan content of 2.7%, a degree of polymerization of 1000, a brightness of 89% ISO, ash content of 0.01%, DCM extract content of 0.02% and an acetifying reaction value (PV value) of 700.

The above examples are preferred embodiments of the disclosure, but embodiments of the disclosure are not limited by the examples, and other any changes, modifications, combinations, substitutions and simplifications made without departing from the spirit and principle of the disclosure are all equivalent substitution modes, and are all included within the protective scope of the disclosure. 

What is claimed is:
 1. A method for producing acetate grade dissolving pulp utilizing poplar residual slabs, comprising: (1) adding an auxiliary agent for prehydrolysis: a liquid ratio is 1:(4˜10), the addition amount of a hydrolysis auxiliary agent is 0.1˜2.0% of the mass of oven dry poplar residual slab, a hydrolysis temperature is 80˜170° C., preservation time is 10˜180 minutes, the hydrolysis auxiliary agent is a mixture obtained by mixing sodium borate, magnesium sulfate, nonylphenol polyoxyethylene ether phosphate and tetrasodium iminodisuccinate in a mass ratio of 1:1:1:1; (2) performing a Soda/AQ cooking I segment; (3) performing a Soda/AQ cooking II segment; (4) performing O_(p)D_(Q)E_(P)P bleaching; and (5) performing enzyme coprocessing: the concentration of pulp is 2˜10%, the temperature is 30˜50° C., treatment time is 60 min, pH=4.0˜7.0, the amount of polyaspartic acid is 0.01%˜1.0% of the mass of oven dry pulp, the amount of OP-13 is 0.001˜0.01% of the mass of oven dry pulp, the amount of OP-15 is 0.001˜0.01% of the mass of oven dry pulp, the amount of endoglucanase is 0.1˜0.6 IU/gpulp, the amount of xylanase is 2˜4 XU/gpulp, and the amount of pectase is 0.1˜10 IU/gpulp.
 2. The method according to claim 1, wherein, the process conditions of the Soda/AQ cooking I segment are as follows: the amount of alkali is 5˜15% of the mass of oven dry pulp based on Na₂O, a liquid ratio is 1:(3˜8), the amount of magnesium hydroxide is 0.5% of the mass of oven dry pulp, the amount of sodium sulfite is 1.0˜10.0% of the mass of oven dry pulp, the amount of AQ is 0.05% of the mass of oven dry pulp, the temperature is 80˜120° C., and preservation time is 10˜180 minutes.
 3. The method according to claim 1, wherein, the process conditions of the Soda/AQ cooking II segment are as follows: the amount of alkali is 5˜25% of the mass of oven dry pulp based on Na₂O, the amount of magnesium hydroxide is 0.5% of the mass of oven dry pulp, the amount of sodium sulfite is 1.0˜10.0% of the mass of oven dry pulp, a liquid ratio is 1:(3˜10), the amount of AQ is 0.05% of the mass of oven dry pulp, the temperature is 140˜180° C., and preservation time is 10˜240 minutes.
 4. The method according to claim 1, wherein, the process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the oxygen pressure of O_(p) bleaching is 0.4˜0.6 Mpa, the concentration of pulp is 10%, the amount of MgSO₄ is 0.6% of the mass of oven dry pulp, the amount of NaOH is 1.0˜3.0% of the mass of oven dry pulp, the amount of H₂O₂ is 0.5˜3.0% of the mass of oven dry pulp, the temperature is 70˜110° C., and preservation time is 60 min.
 5. The method according to claim 1, wherein, the process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the reaction time of Q_(D) bleaching process is 10˜120 minutes, T=30±2° C., the concentration of pulp is 2˜8%, pH=1˜3, the amount of sodium chlorite is 0.2˜6.0% of the mass of oven dry pulp, and the amount of IDS is 0.01˜1.0% of the mass of oven dry pulp.
 6. The method according to claim 1, wherein, the process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the reaction time of the E_(p) extraction process is 30˜180 min, the amount of MgSO₄is 1.0˜5.0% of the mass of oven dry pulp, the amount of NaOH is 0.5˜5.0% of the mass of oven dry pulp, the amount of H₂O₂ is 1.0˜5.0% of the mass of oven dry pulp, the amount of IDS is 0.01˜1.0% of the mass of oven dry pulp, the concentration of pulp is 8%, and the temperature is 70˜95° C.
 7. The method according to claim 1, wherein, the process conditions of O_(p)D_(Q)E_(P)P bleaching are as follows: the reaction time of P bleaching is 60˜180 min, the amount of MgSO₄ is 0.5% of the mass of oven dry pulp, the amount of NaOH is 0.4% of the mass of oven dry pulp, the amount of NaAC is 0.4% of the mass of oven dry pulp, the amount of H₂O₂ is 1.0˜5.0% of the mass of oven dry pulp, the concentration of pulp is 10%, and the temperature is 80˜95° C.
 8. The method according to claim 1, comprising the following steps: (1) adding an auxiliary agent for prehydrolysis: a liquid ratio is 1:6, the addition amount of the auxiliary agent is 0.4% of the mass of oven dry pulp, the temperature is 160° C., preservation time is 60 minutes; (2) performing a Soda/AQ cooking I segment: the amount of alkali is 5% of the mass of oven dry pulp, a liquid ratio is 1:4, the amount of magnesium hydroxide is 0.5% of the mass of absolutely dry pulp, the amount of sodium sulfite is 5.0% of the mass of oven dry pulp, the amount of AQ is 0.05% of the mass of oven dry pulp, T_(max)=100° C.±2° C., and preservation time is 60 minutes; (3) performing a Soda/AQ cooking II segment: the amount of alkali is 18% of the mass of oven dry pulp, the amount of magnesium hydroxide is 0.5% of the mass of oven dry pulp, the amount of sodium sulfite is 3.0% of the mass of oven dry pulp, a liquid ratio is 1:7, the amount of AQ is 0.05% of the mass of oven dry pulp, the maximum temperature is 160° C., and preservation time is 120 minutes; (4) performing O_(p)D_(Q)E_(P)P bleaching: the oxygen pressure of O_(P) bleaching is 0.5 Mpa, the concentration of pulp is 10%, the amount of MgSO₄ is 0.6% of the mass of oven dry pulp, the amount of NaOH is 2.0% of the mass of oven dry pulp, the amount of H₂O₂ is 1.0% of the mass of oven dry pulp, the temperature is 95° C., and preservation time is 60 min; the reaction time of the Q_(D) bleaching process is 60 minutes, T=30±2° C., the concentration of pulp is 4.0%, pH=1.5, the amount of sodium chlorite is 3.0% of the mass of oven dry pulp, and the amount of IDS is 0.05% of the mass of oven dry pulp; the reaction time of the E_(p) extraction process is 90 min, the amount of MgSO₄ is 0.5% of the mass of oven dry pulp, the amount of NaOH is 3.0% of the mass of oven dry pulp, the amount of H₂O₂ is 2.0% of the mass of oven dry pulp, the amount of IDS is 0.05% of the mass of oven dry pulp, the concentration of pulp is 8%, and the temperature is 85° C.; the reaction time of P bleaching is 120 min, the amount of MgSO₄ is 0.5% of the mass of oven dry pulp, the amount of NaOH is 0.4% of the mass of oven dry pulp, the amount of NaAC is 0.4% of the mass of oven dry pulp, the amount of H₂O₂ is 3.0% of the mass of oven dry pulp, the concentration of pulp is 10%, and the temperature is 90° C.; and (5) performing enzyme coprocessing: the concentration of pulp is 5%, the temperature is 40° C., treatment time is 60 min, pH=5.0, the amount of polyaspartic acid is 0.05% of the mass of oven dry pulp, the amount of OP-13 is 0.005% of the mass of oven dry pulp, the amount of OP-15 is 0.005% of the mass of oven dry pulp, the amount of endoglucanase is 0.2 IU/gpulp, the amount of xylanase is 3 XU/gpulp, and the amount of pectase is 3 IU/gpulp.
 9. The method according to claim 1, wherein, in the step (1), the used poplar residual slab is 10˜30 mm in length, the content of pentosan is 22.3˜24.3%, and the content of α-cellulose is 43.0˜45.0%. 